THE MICROBIOLOGY OF THE ATMOSPHERE 



edge; and 90° at right-angles to the wind. (Presentation angle so defined 

 differs from the aeronautical 'angle of incidence' in which at 45°, for 

 example, the leading edge is higher than the trailing edge.) 



The effect of gravity was studied in two sets of experiments. In one 

 set, with the long axis of the slide vertical (parallel with the z-axis) and the 

 surface making various angles with the x, z-plane, the effect of gravity on 

 deposition must be neutral. In the other set, with the long axis of the 

 slide horizontal (parallel with the y-axis) and the surface making various 

 angles with the x, y-plane, the effect of gravity must be positive at angles 

 from 0° up to less than 90°, neutral at 90°, and negative at angles greater 

 than 90° and up to 180° (gravity condition denoted by g+, go, and g_, 

 respectively). Angles greater than 180° represent the back of the slide. 



Preliminary tests showed that trapping efficiency varied in different 

 parts of the slide. Accordingly the slide was divided into five |-cm. zones, 

 denoted : A, B, C, D, and E, respectively, from the leading to the trailing 

 edge (Fig. 13). 



Results of the main series of tests are plotted in Fig. 13, where the 

 efficiency of deposition expected on the gravity theory at 0° for each wind- 

 speed, taking Vs for Lycopodium as 1-76 cm. per sec, is indicated by dotted 

 lines. The observed values below E = o-i per cent are unreliable, but 

 are given to show the trend. Values below E = o-oi per cent, including 

 zero, are all plotted as o-oi per cent as they cannot reasonably be dis- 

 tinguished with the data available. 



The curves obtained probably result from the interaction of several 

 deposition mechanisms: sedimentation, impaction, turbulence, and edge 

 effects. In certain sets of conditions, one or other of the mechanisms can 

 be found acting singly; but for the most part deposition is interpreted as 

 resulting from the simultaneous action of several mechanisms. 



DEPOSITION ON HORIZONTAL SLIDES 



(i) Deposition by sedimentation^ under the influence of gravity alone, 

 is seen on the upper surface of a horizontal slide at the lowest wind-speed 

 tested (conditions denoted by: 0°, 0-5 metres per sec, g+). Here deposition 

 over the slide as a whole was very close to the expected value predicted 

 by the gravity theory (E = Vs/u X 100 = 1*76/50 X 100 = 3-5 per 

 cent), but even at this low wind-speed the bluff edge of the slide, 1-3 mm. 

 thick, caused some edge shadow, shown as a reduced deposition just 

 behind the leading (upwind) edge. That deposition was solely caused by 

 gravity is shown by the absence of deposit on the underside of the hori- 

 zontal slide, and on either side of a vertical slide held parallel with the 

 wind (0°, 05 metres per sec, g_, and go). 



At the wind-speeds more usual outdoors of between i-o and 2-0 

 metres per sec, a surprising effect developed in these wind-tunnel 

 experiments. With gravity positive, the bluff edge of the slide produced an 

 edge shadow deflecting a large proportion of the approaching spores; 



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